A closed link mechanism between a base and a hand may be adopted in a robot arm utilized in an environment where dust must be shut out, such as a clean room or the like. The closed link mechanism uses a motor attached to a base to control the relative angle of two links connected to the base, and can control the position of the hand at a distal end of an arm by changing the shape of a polygon formed by the closed link. This closed link mechanism is suitable for use in the clean room because the motor, which likely gives off dust, does not need to be located at each joint of the robot arm in which the closed link mechanism is comprised. A robot arm in which a closed link mechanism has been adopted is disclosed in Japanese Patent Application Publication No. 2000-208588.